1. Field of the Invention
The present invention relates to a quantum cascade laser using intersubband transition in a quantum well structure.
2. Related Background Art
Light with a mid-infrared wavelength range (for example, wavelength of 5 to 30 μm) is an important wavelength range in the field of spectroscopy analysis. As a high-performance semiconductor light source in this wavelength range, in recent years, quantum cascade lasers (QCL) have gained attention (for example, refer to Patent Documents 1 to 8 and Non-Patent Documents 1 to 7).
A quantum cascade laser is a monopolar type laser element which uses a level structure including subbands formed in a semiconductor quantum well structure and generates light by means of intersubband electron transition, and can realize high efficiency and high-output operations by multistage cascade-coupling of quantum well emission layers which are formed by quantum well structures and become active regions. Further, this cascade coupling of quantum well emission layers is realized by using electron injection layers for injecting electrons into emission upper levels and alternately laminating quantum well emission layers and injection layers.    Patent Document 1: U.S. Pat. No. 5,457,709    Patent Document 2: U.S. Pat. No. 5,745,516    Patent Document 3: U.S. Pat. No. 6,751,244    Patent Document 4: U.S. Pat. No. 6,922,427    Patent Document 5: Japanese Patent Application Laid-Open No. H8-279647    Patent Document 6: Japanese Patent Application Laid-Open No. 2008-177366    Patent Document 7: Japanese Patent Application Laid-Open No. 2008-60396    Patent Document 8: Japanese Patent Application Laid-Open No. H10-4242    Non-Patent Document 1: M. Beck et al., “Continuous Wave Operation of a Mid-Infrared Semiconductor Laser at Room Temperature,” Science Vol. 295 (2002), pp. 301-305    Non-Patent Document 2: J. S. Yu et al., “High-Power Continuous-Wave Operation of a 6 μm Quantum-Cascade Laser at Room Temperature,” Appl. Phys. Lett. Vol. 83 (2003), pp. 2503-2505    Non-Patent Document 3: A. Evans et al., “Continuous-Wave Operation of λ˜4.8 μm Quantum-Cascade Lasers at Room Temperature,” Appl. Phys. Lett. Vol. 85 (2004), pp. 2166-2168    Non-Patent Document 4: A. Tredicucci et al., “A Multiwavelength Semiconductor Laser,” Nature Vol. 396 (1998), pp. 350-353    Non-Patent Document 5: A. Wittmann et al., “Heterogeneous High-Performance Quantum-Cascade Laser Sources for Broad-Band Tuning,” IEEE J. Quantum Electron. Vol. 44 (2008), pp. 1083-1088    Non-Patent Document 6: A. Wittmann et al., “High-Performance Bound-To-Continuum Quantum-Cascade Lasers for Broad-Gain Applications,” IEEE J. Quantum Electron. Vol. 44 (2008), pp. 36-40    Non-Patent Document 7: R. Maulini et al., “Broadband Tuning of External Cavity Bound-to-Continuum Quantum-Cascade Lasers,” Appl. Phys. Lett. Vol. 84 (2004), pp. 1659-1661